This invention relates to drilling of boreholes through subterranean formations, and more particularly to directional control during substantially horizontal drilling through such formations.
The drilling of horizontal boreholes through coal seams in advance of mining for release of methane gas has recently been a subject of intense interest and activity. It is important in such drilling to be able to control the borehole trajectory so that it remains in the coal seam. In an effort to provide this control, considerable work has been done in the area of stabilizers attached to a drill rod string. One such stabilizer is described in U.S. Pat. No. 4,108,256, and a discussion of related work with stabilizers also appears in that patent.
Expandible borehole wall engaging means on a drill rod string are described in U.S. Pat. No. 3,797,589, but are intended for use in advancing the drill string rather than to provide directional control.
There has been a need for a drill rod stabilizer that can provide elevational control to a horizontal borehole being drilled, without the requirement of removing or relocating the stabilizer when a change in borehole trajectory is desired. Such a device is provided by the present invention.